In conventional inter-vehicle distance control systems, the enhancement of response to inter-vehicle distance control between a subject vehicle and a preceding vehicle traveling ahead the subject vehicle and the securement of safety are achieved by taking the following measure.
For example, in the inter-vehicle distance control device described in Patent Document 1 (JP 2567548), when the vehicle speed of the subject vehicle is high and when the inter-vehicle distance is short, the time constant of a filter for relative speed is reduced. When the vehicle speed of the subject vehicle is low and when the inter-vehicle distance is long, the time constant of a filter for relative speed is increased. However, this inter-vehicle distance control system controls the vehicle speed of the subject vehicle based on the relative speed between the subject vehicle and the preceding vehicle so that the inter-vehicle distance is kept constant. The feeling of deceleration at this time is not always comfortable for the driver.
To cope with this, the drive assistance system described in Patent Document 2 (US 2007/0021876, JP 2007-76632A) takes the following measure. This drive assistance system takes, as a driver condition coefficient to be taken as a target (target driver condition coefficient), the driver condition coefficient in a state in which a driver in condition suitable for driving is performing driving operation so as to keep constant the inter-vehicle distance between the preceding vehicle and the subject vehicle. The drive assistance system computes a relative acceleration/deceleration between the subject vehicle and the preceding vehicle to be taken as the target (target relative acceleration/deceleration) based on the target driver condition coefficient and the present driver condition coefficient. The subject vehicle is accelerated or decelerated based on this target relative acceleration/deceleration.
The drive assistance system described in Patent Document 2 does not take the speed of the preceding vehicle into account to determine acceleration/deceleration start timing. Therefore, the driver may not be able to feel the comfortableness of acceleration depending on the driving scene. For this reason, it is proposed in U.S. patent application Ser. No. 12/151,998 filed on May 12, 2008 to determine acceleration/deceleration start timing by using an approximate expression (brake discriminant KdBc) of a corrected distance condition evaluation index KdBc with the speed of the preceding vehicle taken into account so that comfortability can be thereby ensured. The distance condition evaluation index is an evaluation index regarding approaching to and separating from the preceding vehicle.
In this brake discriminant KdBc, the inter-vehicle distance when the preceding vehicle and the subject vehicle come closest to each other is ensured by taking the following measure, as illustrated in FIG. 14. When the inter-vehicle distance D between the preceding vehicle and the subject vehicle is in a short distance range (actual 0 to Dq [m]), the discriminant is substantially unchanged and the deceleration output is increased by reducing an apparent target. For this reason, the target of deceleration output has an inflection point at some midpoint in the process of the inter-vehicle distance D being shortened. As a result, the deceleration output is varied and discomfort is brought to the driver at the time of deceleration.
A brake system for vehicles is also proposed in U.S. patent application Ser. No. 11/805,236, which uses an expression obtained by modifying the following Expression 1 to control deceleration.|−Vr|=2.5×D3×10(|KdB|/10−8)  (Expression 1)
This expression is a formula for computation of distance condition evaluation index, that is evaluation index of approaching/separating relative to the preceding vehicle. In this expression, Vr is target relative speed and KdB is distance condition (approaching/separating) evaluation index.
In this brake system, deceleration is controlled in the following manner. The initial value of distance condition evaluation index when brake operation is started is determined. The subsequent target value of distance condition evaluation index is increased with a constant gradient based on the slope (change rate) of distance condition evaluation index at the start of the brake operation. The deceleration is controlled based on the target relative speed corresponding to this target value and the actual relative speed.
However, the deceleration control using the technique described in the above patent application may not be matched with the feeling of the driver for the following reason.
Assuming that the distance D at the start of control is D0 and the distance condition evaluation index at the start of control be KdB0, the gradient of a tangent of Expression 1 at the start of deceleration control is expressed by Expression 2:dKdB|dD=−30×log10e/D0=−13.03/D0  (Expression 2)
According to Expression 2, an intercept of the tangent is expressed by Expression 3:y-intercept=KdB0+13.03/D0×D0=KdB0+13.03  (Expression 3)
According to the gradient and y-intercept expressed by Expressions 2 and 3 and the initial values D0 and KdB0 at the start of control, the expression of the tangent, that is, the formula for computation of KdB t is expressed by Expression 4:KdBt=−13.03×D/D0+KdB0+13.03  (Expression 4)
When Expression 5 obtained by modifying Expression 4 is substituted into Expression 1, the formula for computation of target relative speed indicated as Expression 6 is obtained:KdBt/10=−1.303×D/D0+0.1×KdB0+1.303  (Expression 5)Vrt=−2.5×D3×10{−1.303×D/D0+0.1×KdB0−6.697}  (Expression 6)
A target relative speed is determined using Expression 6, and deceleration is controlled based on this target relative speed and the actual relative speed. As a result, the vehicle is smoothly decelerated. However, expression 6 means that deceleration is controlled so that Vr=0 when D=0. That is, in deceleration control using Expression 6, deceleration is controlled so that the relative speed becomes 0 when a collision occurs. For this reason, this deceleration control is not matched with the feeling of the driver, and as a result, discomfort is brought to the driver.